XML XSD Schema - Enforce Unique Attribute Values in Schema

@BatteryBackupUnit has the right idea, but the syntax is more like:

<xs:element name="Values">  <xs:complexType>    <xs:sequence>      <xs:element ref="Add" maxOccurs="unbounded"/>    </xs:sequence>  </xs:complexType>  <xs:unique name="UniqueAddKey">    <xs:selector xpath="Add" />     <xs:field xpath="@Key" />   </xs:unique></xs:element>

More on Michael Kay's answer: If your schema (XSD) declares a namespace, you must include this in your selection.xpath. If you are using Microsoft Visual Studio 2010, you may find a namespace automatically declared.

<xs:schema id="MyData"    targetNamespace="http://tempuri.org/MyData.xsd"    elementFormDefault="qualified"    xmlns="http://tempuri.org/MyData.xsd"    xmlns:mstns="http://tempuri.org/MyData.xsd"    xmlns:xs="http://www.w3.org/2001/XMLSchema">    ...    <xs:unique name="UniqueAddKey">        <xs:selector xpath="mstns:Add" />         <xs:field xpath="@Key" />     </xs:unique></xs:schema>

you can achieve this by using xs:unique

<xs:element name="Instrument">  <xs:complexType>   <xs:all>    <xs:unique name="ModuleId">      <xs:selector xpath="./*" />       <xs:field xpath="@id" />     </xs:unique>   </xs:all>  </xs:complexType></xs:element>

The above example will enforce a unique attribute "id" for all Instrument Elements.there's also xs:key, which can be used to establish a Primary Key - Foreign Key relationship:http://www.datypic.com/books/defxmlschema/chapter17.html